Facsimile systems are widely used to transmit images, typically of documents, from one location to another. Conventionally, a document is scanned by a "fax machine" to convert it into facsimile data which is then transmitted over a communications channel, typically a voice channel on the Public Switched Telephone Network (PSTN). The document is then reconstructed into hard copy from the transmitted facsimile data by a similar facsimile machine located at the other end of the communications channel.
Standard communications protocols have been developed for transmission of facsimile data. Older analog protocols, such as CCITT Group 1 and Group 2, are now obsolete. The digital CCITT Group 3 protocol, widely used by conventional office and home facsimile machines, uses digital modems transmitting digital facsimile data at speeds up to 9600 baud over conventional PSTN telephone lines. The Group 3 protocol includes handshaking and signal quality evaluation features which enable facsimile machines to step down the communication baud rate if the communications channel exhibits poor quality. A CCITT Group 4 protocol has also been established to provide less error-prone communication between facsimile machines over public data networks other than the PSTN.
Personal computers (PCs) may also employ hardware and software which emulate the functions of a facsimile machine. A facsimile application program resident in the PC may create and edit "fax files," i.e., data files including facsimile data representing facsimile images communicated to and from other facsimile stations. This facsimile data is communicated to and from a so-called "fax modem" plugged into an expansion slot of the PC. The fax modem is connected to the PSTN in a manner similar to a standard facsimile machine, and sends and receives signals over the PSTN according to a standard facsimile data transfer protocol, conventionally the Group 3 protocol.
Because of the increased mobility it offers, cellular radiotelephone technology is increasingly supplanting the PSTN for many communications applications. Mobile cellular radiotelephones are now a common sight in businesses, home, and automotive environments. Personal computers have also been adapted for use with cellular radiotelephone communications systems, exchanging the wireline modem for a similar device acting over a cellular radiotelephone voice or data channel.
A major obstacle to implementation of facsimile communications over cellular radiotelephone communications systems is the generally lower bandwidth, lower signal quality, and higher communications costs associated with cellular radiotelephone channels in comparison to conventional PSTN channels. For example, due to the cellular nature of cellular radiotelephone communications systems, communications between subscribers in the system may be abruptly interrupted as subscribers move between cells in the system. While such interruptions are normally tolerable in voice applications, they generally are unacceptable for facsimile communication, causing the loss of significant amount of data and corruption of the facsimile image being transmitted. Similarly, cochannel interference which may be tolerable in a voice application may cause unacceptable corruption of facsimile data transmitted over a cellular radiotelephone channel.
The Group 3 protocol used by conventional facsimile machines and personal computer fax modems is generally ill-suited for cellular radiotelephone communications because the Group 3 protocol is designed for the relatively reliable, relatively high-bandwidth and relatively low-cost PSTN. As described above, the Group 3 protocol is generally limited to stepping down the baud rate of the communications between facsimile machines when the communications channel between the machines is poor. Such a response can be ineffective in dealing with service interruptions and high levels of co-channel interference, however. In addition, communicating facsimile data over a cellular radiotelephone voice channel according to the Group 3 protocol can be economically inefficient, as time on a cellular radiotelephone channel tends to be relatively expensive in comparison to time on a conventional PSTN voice channel. Although Group 3 includes some data compression to remove redundant data in a particular facsimile image, a significant amount of redundant data may be still be transmitted over the communications channel. For example, if the same document is to be transmitted to multiple facsimile stations, the entire document, including repetitive elements such as cover page form elements, typically must be sent across a cellular radiotelephone link to each recipient.
A system for providing more reliable radio transmission of facsimile data using packet data protocols with checking and retransmission features is described in U.S. Pat. No. 5,282,238 to Berland. Although such a system can cut down on transmission errors, it may not fully optimize communication of facsimile data over the radio link. For example, a significant amount of redundant information may still be communicated over the radio link, such as cover page form elements or multiple transmissions of the same image. When combined with the retransmission of data which may occur because of the use of a more robust data transmission protocol, such a system may prove to be prohibitively costly for some facsimile users. Moreover, such a system is designed to interface with a standard facsimile machine or a fax modem which produces data according to a standard facsimile protocol such as the Group 3 protocol, and thus may not realize data transfer efficiencies which may be gained from directly interfacing to a facsimile application program resident on a remote computer.